In the motorized actuation of functional units using an actuating element in which the function of the functional unit changes, e.g., in proportion to the position of the actuating element, it is frequently necessary to reference the position of the actuating element so that the precise position of the actuating element is known in a control unit. An example of this is the motorized actuation of a clutch in which the position of a clutch lever determines the torque transferability of the clutch. A precise knowledge of the torque transferability of the clutch is important to precisely control or regulate starting and shifting.
Referencing is especially essential when for example an electric motor is used as the actuator that drives the actuating element via a shaft whose rotary position is detected with an incremental sensor. The incremental sensor cannot detect absolute positions of the actuating element, but merely changes in position so that it is necessary to determine the absolute position of the actuating element or the rotary position of the output shaft of the electric motor by means of referencing.
A control element for a motor vehicle friction clutch is known from DE 44 33 825 C2 in which the output shaft of an electric motor drives a segment gear on which an actuating element for the clutch is mounted. The rotational range of the segment gear is limited by stops that can be approached for referencing. In this manner, the actuating element's absolute position can be determined from which the displacement path can be detected by means of an incremental sensor.
A device for shifting a gear is known from DE 100 27 330 A1 in which an electric motor whose rotary position is detected by means of an incremental sensor drives an actuating element of the transmission that can be moved into an end position against a stop. The actuating element is additionally equipped with a surface profile that engages with a spring-loaded pickup component. The profile of the actuating element is such that the force required to displace the actuating element depends on the relative position between the surface profile and the pickup component. The slope of shape surfaces of the profile can be such that the force applied by the elastic pretension of the pickup component on the actuating element is greater in sections than the holding torque of the electric motor so that one can determine if the actuating element is within a predetermined control range when a small voltage is applied to the electric motor.
With referencing that uses stops and thereby recognizes that the actuating element does not move despite the force exerted by the actuator on the actuating element, there is a danger of incorrectly identifying a blockage as a reference position in a transmission path in which the actuating element is located. Particularly when actuating clutches, this can cause hazardous operating conditions since the vehicle may for example unintentionally drive away.